The present invention generally relates to a process and a plant to apply one or more compounds to a substrate and, in particular, a process and plant intended to be used for printing with one or more colours on substrates of different kinds.
Currently various techniques are known for generally applying one or more compounds to a substrate or, in particular, printing one or more colours on substrates of different kinds, such as paper, fabrics, synthetic films or the like. In general, printing can be effected by directly applying impressions of various colours to the substrate intended to receive these or, alternatively, using xe2x80x9ctransferxe2x80x9d techniques, according to which the impressions of each of the different colours are first applied to a transfer medium which is then placed in contact with the substrate of destination.
Irrespective of the possible advantages or disadvantages of each printing technique, the choice of one of these two techniques is also determined by the type of material which forms the substrate to be printed and/or the type of material, such as ink or the like, to be applied to the substrate.
A specific example of the application of the present invention is printing on fabrics or the like. Reference to this particular field of application must be considered merely as an unlimited example of the present invention.
Traditionally, printing on fabrics is effected using vinyl-chloride resin based inks, such as polyvinylchloride (PVC), which are applied directly to the fabric using screen printing techniques. These methods generally allow a high level of productivity and satisfactory quality of print to be obtained.
Nonetheless, the presence of vinyl-chloride resins in these type of inks now makes this technique unacceptable from the viewpoint of safeguarding the environment. In fact, these resins may cause dioxin to form during the step to dispose of the fabric as refuse. Moreover, printing machinery using these types of inks must be cleaned using solvents that may be harmful to the environment. From the viewpoint of national and international regulations, increasingly sensitive to problems of safeguarding the environment, there is therefore a tendency to gradually limit the use of these inks through time, until products manufactured in this way are totally eliminated from the market.
Direct application on the substrate of compounds without vinyl-chloride resins, such as inks generally composed of volatile solvents, for instance water-based inks, nonetheless causes various problems.
Firstly, productivity is drastically reduced, especially if several colours or several compounds are to be applied to the substrate. In fact, to obtain an acceptable quality of print, the solvent of the ink of a specific colour (or a specific compound) must be removed before the subsequent colour (or compound) is applied.
In the case of prints with different colours, this is implemented by setting up a drying station immediately downstream from the printing station of each colour and keeping the substrate to be printed at a high temperature for a preset time in the drying station before sending it to the subsequent printing station, where another colour is applied.
Moreover, the substrate subjected to drying must in any case be left to cool before being sent to the subsequent printing station, since if the substrate were still hot this would cause coagulation of the ink present on screen of the next screen printing station.
If the substrate is formed of fabrics or the like, direct application of water-based inks using the screen printing technique has to date proved to be somewhat unsuitable for printing on dark coloured fabrics due to the poor covering effect that these inks can guarantee when applied directly to fabrics in a liquid state.
In general, the object of the present invention is to provide a process and a plant that allow one or more compounds to be applied to a substrate overcoming the problems of the prior art.
Another object of the present invention is to provide a plant of the aforesaid type particularly small in size and which consumes less power than plants of the known kind.
More specifically, another object of the present invention is to provide a process and plant that allow the application to a substrate of compounds with no vinyl-chloride resins whatsoever.
Yet another object of the present invention is to provide a process and plant that allow improvement of the productivity of the techniques for application to a substrate of compounds with volatile solvents.
Another more specific object of the present invention is to provide a process and a plant that allow particularly efficient application of compounds comprising volatile solvents to specific types of substrate, such as fabrics or the like.
These objects are achieved according to the present invention thanks to a process for the application of one or more compounds on a substrate, the process comprising the steps of:
a) applying a preset amount of a mixture to a transfer medium, the mixture comprising one or more compounds in the form of a solution, emulsion or suspension and being essentially in a liquid state;
b) heating the transfer medium to a preset temperature to obtain at least a partial coagulation of the portion of the mixture in contact with the transfer medium, the other portion of mixture remaining in its essentially liquid state; and
c) placing the transfer medium in contact with the substrate by exerting a given pressure so that the coagulated portion of the mixture is placed on the substrate.
In this way it is possible to obtain efficient covering of the substrate, because a part of the mixture that is transferred to the substrate has already coagulated or, in other words, has already been deprived of part of the solvent at the time of application.
By obtaining a surface layer that has already coagulated specifically allows the immediate application of a subsequent compound, for example the portion of an impression implemented with a product having different composition or colour specifications, without having to perform any intermediate drying steps of the previous compound and without the risk of unwanted mixing of compounds, such as smudging of colours if the compounds are inks, around the edges of adjacent areas.
According to another aspect of the invention, the process also comprises cooling of the transfer medium before application of another preset amount of the mixture to the same support. If a mixture comprises a volatile solvent, by cooling the transfer medium this prevents the next compound from being coagulated by this still hot support at the time it is applied to this transfer medium. This is particularly advantageous if the screen-printing technique is used to deposit the compound on the transfer medium, as this prevents the compound from coagulating on the printing screen and clogging the holes through which the compound passes.
In the specific case in which the substrate is formed of a fabric or the like, during transfer of the compound from the support to the fabric below, the part of the mixture that is still liquid is free to penetrate deep into the fabric, thus consolidating adhesion of the compound to the fabric.
The invention also concerns a plant for applying one or more compounds to a substrate, of the type comprising one or more stations, each of which provided with means to apply an essentially liquid mixture comprising at least one of the compounds to a transfer medium in order to subsequently obtain the application of the compound on the substrate, characterised in that each of the stations comprises means for adjusting the temperature of the relevant transfer medium and means for adjusting the contact pressure between the relevant transfer medium and the substrate on which each compound is to be applied.
In the specific case of a plant for the application of compounds based on volatile solvents, a plant with a particularly small size is thus obtained as it is no longer necessary to provide a drying station for the mixture applied to the substrate below each application station.
In the specific case of plants for printing on fabric substrates, taking into account that plants of the known type can have up to ten printing stations (one for each colour) it is easy to understand the advantages of a plant implemented according to the present invention.
According to a possible embodiment of the invention, the transfer medium consists of a closed belt that is made to rotate between at least two cylinders.
In this case at least a first cylinder is provided to heat the transfer medium in a controlled manner and to exert, also in a controlled manner, the contact pressure between the transfer medium and the substrate. A second cylinder cools the transfer medium in a controlled manner and the mixture is applied to the transfer medium at the level of this second cylinder.
According to another possible embodiment, the transfer medium is formed of a closed belt that rotates between at least three cylinders.
Similarly to the previous possible embodiment, the first cylinder is designed to heat the transfer medium in a controlled manner and exert, in a controlled manner, contact pressure between the transfer medium and the substrate. A second cylinder is used to cool the transfer medium in a controlled manner while, differently to the previous possible embodiment, at least a third cylinder is provided, at the level of which the mixture is placed on the transfer medium.
In both cases, the mixture can be placed for example on the transfer medium by means of a screen printing device.